This is a very simple calculator (using Microsoft Excel) to calculate the percentage of your GTA IV game completion. [If possible, I'll make an executable (.exe) file for this later.] Just for fun!
There're two versions: ".xls" for Office 2003 or older, ".xlsx" for Office 2007 or newer.

For more details, go to "gta4.net" > "100% Checklist".





Notes:

[1] I noticed that the game completion was 0.70% after completing the first mission (The Cousins Bellic). Therefore, there're 86 missions that counts, since storyline missions make up exactly 60% of the game completion. (60% / 86 = 0.6976744%) (Maybe the number of missions isn't 86, so you need to try several numbers around the actual number).

[2] You need to meet 10 random characters, 0.5% for each PERSON, not for each ENCOUNTER.

[3] The 6 items (1.66667% for each) about friends are:

 i) Achieve 70% ‘like stat’ for Brucie
 ii) Achieve 60% ‘like stat’ for Jacob
 iii)Achieve 75% ‘like stat’ for Packie
 iv) Do all 7 activities with Brucie
 v) Do all 6 activities with Jacob
 vi) Do all 6 activities with Packie

[4] The 4 activities (1.25% for each) are:

 i) Win a game of pool
 ii) Win a game of darts
 iii)Win a FULL game of bowling (10 rounds, not 5 rounds)
 iv) Beat the QUB3D high score (reach 11, 000 points)

Enjoy it. The result should be exact. However, if you find any mistakes, please leave a comment here, thank you.